“The finding was very surprising to us initially. Most people think new neurons mean better memory,” said Sheena Josselyn, a neuroscientist from the Hospital for Sick Children in Toronto, who led the study.

Until now, many scientists have thought long-term memories might be tied to language because children typically start forming long-term memories at about the same time they start talking.

“But the really weird thing is that most animals show infantile amnesia too,” said Josselyn.

“So the development of language can’t be the whole explanation.”

Inspired by observations of their own toddler, Josselyn and her husband, study coauthor Paul Frankland, started to question why young children could not retain memories of situations or events.

The memories involve the hippocampus, the only part of the brain which normally produces new neurons, which scientists believe is vital to make memories.

But the production of these new cells tapers off in childhood.

“That’s exactly when we start to be able to form long-term memories,” Josselyn said.

Her study of mice, which like humans fail to store early memories, showed the decline in the production of new neurons as the rodents grew up matched their ability to remember frightening situations.

Josselyn said: “More neurons increase the capacity to learn new memories in the future

“But memory is based on a circuit, so if you add to this circuit, it makes sense that it would disrupt it.”

She believes the newly added neurons could have a useful role in clearing old memories and making way for new ones.